5-Chloro-N-({(5S)-2-oxo-3-[4-(5,6-dihydro-4H-[1,2,4]triazin-1-yl)phenyl]-1,3-oxazolidin-5-yl}methyl)thiophene-2-carboxamide methanesulfonate of the following formula (1) has been known as an inhibitor of blood coagulation factor Xa and used for treating and preventing thrombosis, myocardial infarction, arteriosclerosis, inflammation, stroke, angina pectoris, recurrent stricture after angioplasty, and thromboembolism such as intermittent claudication (see U.S. Pat. No. 8,178,25).

According to the guidelines and regulations which are promulgated by drug regulatory authorities in various countries, the stability of a drug crystal is required for obtaining the market approval of the drug. Accordingly, there has been a need for developing a novel crystalline form of 5-chloro-N-({(5S)-2-oxo-3-[4-(5,6-dihydro-4H-[1,2,4]triazin-1-yl)phenyl]-1,3-oxazolidin-5-yl}methyl)thiophene-2-carboxamide methanesulfonate which has an excellent dissolution rate and stability even in a high temperature and humid environments.